1. Field of the Invention
The present invention relates to surface acoustic wave motors.
2. Discussion of the Related Art
Surface acoustic wave motors are described, for example, in U.S. Pat. No. 4,562,374 by T. Sashida. The working principle of such a motor is illustrated in FIG. 1 of the Sashida patent, reproduced in the attached FIG. 1. A progressive surface wave of the Rayleigh type produced at the surface of a fixed elastic body 1 causes ripples at the surface of the body. If a mobile part 2 is pressed against body 1, part 2 is driven by the displacement of apexes A-A' of these ripples. Apexes A-A' run along an elliptic path Q and their transverse displacement speed is associated with the oscillation frequency and the amplitude of their displacement. In practice, the excitation frequency of the surface waves must be close to a resonance frequency of the body at the surface of which it is desired to produce these waves and this frequency can be modified only within a small range if it is desired to maintain a non-negligible amplitude (for example, approximately 10 .mu.m) of the apexes of the ripples. A slight frequency mismatch with respect to the resonance frequency causes a major amplitude variation. Thus, the relation between the speed of movement in the direction of arrow N of apexes A and A' and the excitation frequency is a complex non-linear relation.
The surface acoustic wave motor of U.S. Pat. No. 4,562,374 has the advantages of being very little noisy (since the frequency of the excitation signal is within a non-audible range, for example, 20 to 100 kHz), relatively performing, small-sized and light for a determined torque, and of having a high hold torque (that is, when no signal is applied, the mobile part 2 is applied against the fixed part 1 and a non-negligible friction force is present between both parts).
However, the drawback of such a motor is that its speed can be controlled only by using a system including a sensor and servo-controlled loop. The same system is also necessary if it is desired to control the position of this motor. The use of such sensors and servo-controlled loops results in a very complex and very expensive motor, which counterbalances its basic advantages.